Mapping an input file into memory and then directly parsing data from the mapped memory pages can be a convenient and efficient way to read data from files.
However, this practice also seems fundamentally unsafe unless you can ensure that no other process writes to a mapped file, because even the data in private read-only mappings may change if the underlying file is written to by another process. (POSIX e.g. doesn't specify "whether modifications to the underlying object done after the MAP_PRIVATE mapping is established are visible through the MAP_PRIVATE mapping".)
If you wanted to make your code safe in the presence of external changes to the mapped file, you'd have to access the mapped memory only through volatile pointers and then be extremely careful about how you read and validate the input, which seems impractical for many use cases.
Is this analysis correct? The documentation for memory mapping APIs generally mentions this issue only in passing, if at all, so I wonder whether I'm missing something.
It is not really a problem.
Yes, another process may modify the file while you have it mapped, and yes, it is possible that you will see the modifications. It is even likely, since almost all operating systems have unified virtual memory systems, so unless one requests unbuffered writes, there's no way of writing without going through the buffer cache, and no way without someone holding a mapping seeing the change.
That isn't even a bad thing. Actually, it would be more disturbing if you couldn't see the changes. Since the file quasi becomes part of your address space when you map it, it makes perfect sense that you see changes to the file.
If you use conventional I/O (such as read), someone can still modify the file while you are reading it. Worded differently, copying file content to a memory buffer is not always safe in presence of modifications. It is "safe" insofar as read will not crash, but it does not guarantee that your data is consistent.
Unless you use readv, you have no guarantees about atomicity whatsoever (and even with readv you have no guarantee that what you have in memory is consistent with what is on disk or that it doesn't change between two calls to readv). Someone might modify the file between two read operations, or even while you are in the middle of it.
This isn't just something that isn't formally guaranteed but "probably still works" -- on the contrary, e.g. under Linux writes are demonstrably not atomic. Not even by accident.
The good news:
Usually, processes don't just open an arbitrary random file and start writing to it. When such a thing happens, it is usually either a well-known file that belongs to the process (e.g. log file), or a file that you explicitly told the process to write to (e.g. saving in a text editor), or the process creates a new file (e.g. compiler creating an object file), or the process merely appends to an existing file (e.g. db journals, and of course, log files). Or, a process might atomically replace a file with another one (or unlink it).
In every case, the whole scary problem boils down to "no issue" because either you are well aware of what will happen (so it's your responsibility), or it works seamlessly without interfering.
If you really don't like the possibility that another process could possibly write to your file while you have it mapped, you can simply omit FILE_SHARE_WRITE under Windows when you create the file handle. POSIX makes it somewhat more complicated since you need to fcntl the descriptor for a mandatory lock, which isn't necessary supported or 100% reliable on every system (for example, under Linux).
In theory, you're probably in real trouble if someone does
modify the file while you're reading it. In practice: you're
reading characters, and nothing else: no pointers, or anything
which could get you into trouble. In practice... formally,
I think it's still undefined behavior, but it's one which
I don't think you have to worry about. Unless the modifications
are very minor, you'll get a lot of compiler errors, but that's
about the end of it.
The one case which might cause problems is if the file was
shortened. I'm not sure what happens then, when you're reading
beyond the end.
And finally: the system isn't arbitrarily going to open and
modify the file. It's a source file; it will be some idiot
programmer who does it, and he deserves what he gets. In no
case will your undefined behavior corrupt the system or other
peoples files.
Note too that most editors work on a private copy; when the
write back, they do so by renaming the original, and creating
a new file. Under Unix, once you've opened the file to mmap
it, all that counts is the inode number. And when the editor
renames or deletes the file, you still keep your copy. The
modified file will get a new inode. The only thing you have to
worry about is if someone opens the file for update, and then
goes around modifying it. Not many programs do this on text
files, except for appending additional data to the end.
So while formally, there's some risk, I don't think you have to
worry about it. (If you're really paranoid, you could turn off
write authorisation while you're mmaped. And if there's
really an enemy agent out to get your, he can turn it right back
on.)
Related
I need to learn how to update a file concurrently without blocking other threads. Let me explain how it should work, needs, and how I think it should be implemented, then I ask my questions:
Here is how the worker works:
Worker is multithreaded.
There is one very large file (6 Terabyte).
Each thread is updating part of this file.
Each write is equal to one or more disk blocks (4096 bytes).
No two worker write at same block (or same group of blocks) at the same time.
Needs:
Threads should not block other blocks (no lock on file, or minimum possible number of locks should be used)
In case of (any kind of) failure, There is no problem if updating block corrupts.
In case of (any kind of) failure, blocks that are not updating should not corrupts.
If file write was successful, we must be sure that it is not buffered and be sure that actually written on disk (fsync)
I can convert this large file to as many smaller files as needed (down to 4kb files), but I prefer not to do that. Handling that many files is difficult, and needs a lot of file handles open/close operations, which has negative impact on performance.
How I think it should be implemented:
I'm not much familiar with file manipulation and how it works at operating system level, but I think writing on a single block should not corrupt other blocks when errors happen. So I think this code should perfectly work as needed, without any change:
char write_value[] = "...4096 bytes of data...";
int write_block = 12345;
int block_size = 4096;
FILE *fp;
fp = fopen("file.txt","w+");
fseek(fp, write_block * block_size, SEEK_SET);
fputs(write_value, fp);
fsync(fp);
fclose(fp);
Questions:
Obviously, I'm trying to understand how it should be implemented. So any suggestions are welcome. Specially:
If writing to one block of a large file fails, what is the chance of corrupting other blocks of data?
In short, What things should be considered on perfecting code above, (according to the last question)?
Is it possible to replace one block of data with another file/block atomically? (like how rename() system call replaces one file with another atomically, but in block-level. Something like replacing next-block-address of previous block in file system or whatever else).
Any device/file system/operating system specific notes? (This code will run on CentOS/FreeBSD (not decided yet), but I can change the OS if there is better alternative for this problem. File is on one 8TB SSD).
Threads should not block other blocks (no lock on file, or minimum possible number of locks should be used)
Your code sample uses fseek followed by fwrite. Without locking in-between those two, you have a race condition because another thread could jump in-between. There are three reasonable solutions:
Use flockfile, followed by regular fseek and fwrite_unlocked then funlock. Those are POSIX-2001 standard
Use separate file handles per thread
Use pread and pwrite to do IO without having to worry about the seek position
Option 3 is the best for you.
You could also use the asynchronous IO from <aio.h> to handle the multithreading. It basically works with a thread-pool calling pwrite on most Unix implementations.
In case of (any kind of) failure, There is no problem if updating block corrupts
I understand this to mean that there should be no file corruption in any failure state. To the best of my knowledge, that is not possible when you overwrite data. When the system fails in the middle of a write command, there is no way to guarantee how many bytes were written, at least not in a file-system agnostic version.
What you can do instead is similar to a database transaction: You write the new content to a new location in the file. Then you do an fsync to ensure it is on disk. Then you overwrite a header to point to the new location. If you crash before the header is written, your crash recovery will see the old content. If the header gets written, you see the new content. However, I'm not an expert in this field. That final header update is a bit of a hand-wave.
In case of (any kind of) failure, blocks that are not updating should not corrupts.
Should be fine
If file write was successful, we must be sure that it is not buffered and be sure that actually written on disk (fsync)
Your sample code called fsync, but forgot fflush before that. Or you set the file buffer to unbuffered using setvbuf
I can convert this large file to as many smaller files as needed (down to 4kb files), but I prefer not to do that. Handling that many files is difficult, and needs a lot of file handles open/close operations, which has negative impact on performance.
Many calls to fsync will kill your performance anyway. Short of reimplementing database transactions, this seems to be your best bet to achieve maximum crash recovery. The pattern is well documented and understood:
Create a new temporary file on the same file system as the data you want to overwrite
Read-Copy-Update the old content to the new temporary file
Call fsync
Rename the new file to the old file
The renaming on a single file system is atomic. Therefore this procedure will ensure after a crash, you either get the old data or the new one.
If writing to one block of a large file fails, what is the chance of corrupting other blocks of data?
None.
Is it possible to replace one block of data with another file/block atomically? (like how rename() system call replaces one file with another atomically, but in block-level. Something like replacing next-block-address of previous block in file system or whatever else).
No.
I know this is a bit of theoretical question but haven't got any satisfactory answer yet. So thought to put this question here.
I have multiple C++ processes (would also like to know thread behaviour) which contend to replace the same file at the same time. How much is it safe to do in Linux (Using Ubuntu 14.04 and Centos 7)? Do I need to put locks?
Thanks in advance.
The filesystems of Unix-based OS's like Linux are designed around the notion of inodes, which are internal records describing various metadata about the file. Normally these aren't interacted with directly by users or programs, but their presence gives these filesystems a level of indirection that allows them to provide some useful semantics that other OS's (read: Windows) cannot.
filename --> inode --> data
In particular, when a file gets deleted, what's actually happening is the separation of the file's inode from its filename; not (necessarily) the deletion of the file's data itself. That is, the file and its contents can continue to exist (albeit invisibly, from the user's point of view) until all processes have closed their file-handles that were open on that file; once the inode is no longer accessible to any process, only then will the filesystem actually mark the file's data-blocks as free and available-for-reuse. In the meantime, the filename becomes available for another file's inode (and data) to be associated with, even though the old file's inode/data still technically exists.
The upshot of that is that under Linux it's perfectly valid to delete (or rename) a file at any time, even if other threads/processes are in the middle of using it; your delete will succeed, and any other programs that have that file open at that instant can simply continue reading/writing/using it, exactly as if it hadn't been deleted. The only thing that is different is that the filename will no longer appear in its directory, and when they call fclose() (or close() or etc) on the file, the file's data will go away.
Since doing mv new.txt old.txt is essentially the same as doing a rm old.txt ; mv new.txt old.txt, there should be no problems with doing this from multiple threads without any synchronization. (note that the slightly different situation of having multiple threads or processes opening the same file simultaneously and writing into it at the same time is a bit more perilous; nothing will crash, but it would be easy for them to overwrite each other's data and corrupt the file, if they aren't careful)
It depends a lot on exactly what you're going to be doing and how you're using the files. In general, in Unix/Posix systems like Linux, all file calls will succeed if multiple processes make them, and the general way the OS handles contention is "the last one to do something wins". Essentially, all modifications to the filesystem are serialized, so the filesystem is always in a consistent state. But otherwise it's a free-for-all.
There are a lot of details here though. There's flags used in opening a file like O_EXCL that can result in failure if another process did it first (a sort of lock). There's advisory (aka, nobody is forced by the OS to pay attention to them) locking systems like flock (try typing man 2 flock to learn more) for file contents. There are more Linux specific mandatory locking system.
And there are also details like "What happens if someone deleted a file I have open?" that the other answer explains correctly and well.
And lastly, there's a whole mess of detail surrounding whether it's guaranteed that any particular change to the filesystem is recorded for all eternity, or whether it has a chance of disappearing if someone flicks the power switch. And that's a mess-and-a-half once you really dive into it, between dodgy hardware that lies to the OS about things to the confusing morass of different Linux system calls covering different aspects of this problem, often entering Linux from different eras of Unix/Posix history and interacting with each other in strange and arcane ways.
So, an answer to your very general and open-ended question is going to have to necessarily be vague, abstract, and hand-wavey.
We're writing a C++/Objective C app, runnable on OSX from versions 10.7 to present (10.11).
Under windows, there is the concept of a shadow file, which allows you read a file as it exists at a certain point in time, without having to worry about other processes writing to that file in the interim.
However, I can't find any documentation or online articles discussing a similar feature in OS X. I know that OS X will not lock a file when it's being written to, so is it necessary to do something special to make sure I don't pick up a file that is in the middle of being modified?
Or does the Journaled Filesystem make any special handling unnecessary? I'm concerned that if I have one process that is creating or modifying files (within a single context of, say, an fopen call - obviously I can't be guaranteed of "completeness" if the writing process is opening and closing a file repeatedly during what should be an atomic operation), that a reading process will end up getting a "half-baked" file.
And if JFS does guarantee that readers only see "whole" files, does this extend to Fat32 volumes that may be mounted as external drives?
A few things:
On Unix, once you open a file, if it is replaced (as opposed to modified), your file descriptor continues to access the file you opened, not its replacement.
Many apps will replace rather than modify files, using things like -[NSData writeToFile:atomically:] with YES for atomically:.
Cocoa and the other high-level frameworks do, in fact, lock files when they write to them, but that locking is advisory not mandatory, so other programs also have to opt in to the advisory locking system to be affected by that.
The modern approach is File Coordination. Again, this is a voluntary system that apps have to opt in to.
There is no feature quite like what you described on Windows. If the standard approaches aren't sufficient for your needs, you'll have to build something custom. For example, you could make a copy of the file that you're interested in and, after your copy is complete, compare it to the original to see if it was being modified as you were copying it. If the original has changed, you'll have to start over with a fresh copy operation (or give up). You can use File Coordination to at least minimize the possibility of contention from cooperating programs.
I usually use two methods to write to files, either with Qt's QFile or STL's fstream.
I have a long-running (several minutes) simulation which logs data to a file. Performance-wise and design-wise, is it a good idea to:
Keep it open the whole time
Close and open on every write
Somewhere in-between (1) and (2)
Several question on here address this issue (for Perl, for fopen), but I didn't see any discussion of QFile and fstream. The previous answers suggest to keep it open (option 1). Is this still the cast for QFile and fstream?
Performance wise, it would definitely be better to keep it open for the life of the application, just because less work opening and closing files means less time spent doing things that don't move the application closer to completion, which will slow the program down. As for design, just make sure to close the file before the application terminates.
QFile and fstream probably use fopen, fwrite etc under the hood (although it is of course implementation dependent). So I would bet that anything applying to FILE*s would apply to QFiles and fstreams.
This may be dependent on your libc implementation but fstream is generally uses memory-mapped files. These are generally very efficient, and only main memory or swap when a page of data is written to.
If you are running a 32-bit system and these files are very large or very numerous then you could have issues with exhausting the virtual address space (on windows ~2GB might cause such problems). Seeing as you are simply logging this seems quite unlikely.
But simply closing the files might make it worse in that case because then the virtual address space could become fragmented.
I would advise that it is best to leave the files open at all times unless you think you will run into the issues above. If you are memory constrained then flushing the data will reduce the physical memory requirements.
Using C or C++, After I decrypt a file to disk- how can I guarantee it is deleted if the application crashes or the system powers off and can't clean it up properly? Using C or C++, on Windows and Linux?
Unfortunately, there's no 100% foolproof way to insure that the file will be deleted in case of a full system crash. Think about what happens if the user just pulls the plug while the file is on disk. No amount of exception handling will protect you from that (the worst) case.
The best thing you can do is not write the decrypted file to disk in the first place. If the file exists in both its encrypted and decrypted forms, that's a point of weakness in your security.
The next best thing you can do is use Brian's suggestion of structured exception handling to make sure the temporary file gets cleaned up. This won't protect you from all possibilities, but it will go a long way.
Finally, I suggest that you check for temporary decrypted files on start-up of your application. This will allow you to clean up after your application in case of a complete system crash. It's not ideal to have those files around for any amount of time, but at least this will let you get rid of them as quickly as possible.
Don't write the file decrypted to disk at all.
If the system is powerd off the file is still on disk, the disk and therefore the file can be accessed.
Exception would be the use of an encrypted file system, but this is out of control of your program.
I don't know if this works on Windows, but on Linux, assuming that you only need one process to access the decrypted file, you can open the file, and then call unlink() to delete the file. The file will continue to exist as long as the process keeps it open, but when it is closed, or the process dies, the file will no longer be accessible.
Of course the contents of the file are still on the disk, so really you need more than just deleting it, but zeroing out the contents. Is there any reason that the decrypted file needs to be on disk (size?). Better would just to keep the decrypted version in memory, preferably marked as unswappable, so it never hits the disk.
Try to avoid it completely:
If the file is sensitive, the best bet is to not have it written to disk in a decrypted format in the first place.
Protecting against crashes: Structured exception handling:
However, you could add structured exception handling to catch any crashes.
__try and __except
What if they pull the plug?:
There is a way to protect against this...
If you are on windows, you can use MoveFileEx and the option MOVEFILE_DELAY_UNTIL_REBOOT with a destination of NULL to delete the file on the next startup. This will protect against accidental computer shutdown with an undeleted file. You can also ensure that you have an exclusively opened handle to this file (specify no sharing rights such as FILE_SHARE_READ and use CreateFile to open it). That way no one will be able to read from it.
Other ways to avoid the problem:
All of these are not excuses for having a decrypted file on disk, but:
You could also consider writing to a file that is larger than MAX_PATH via file syntax of \\?\. This will ensure that the file is not browsable by windows explorer.
You should set the file to have the temporary attribute
You should set the file to have the hidden attribute
In C (and so, I assume, in C++ too), as long as your program doesn't crash, you could register an atexit() handler to do the cleanup. Just avoid using _exit() or _Exit() since those bypass the atexit() handlers.
As others pointed out, though, it is better to avoid having the decrypted data written to disk. And simply using unlink() (or equivalent) is not sufficient; you need to rewrite some other data over the original data. And journalled file systems make that very difficult.
A process cannot protect or watch itself. Your only possibility is to start up a second process as a kind of watchdog, which regularly checks the health of the decrypting other process. If the other process crashes, the watchdog will notice and delete the file itself.
You can do that using hearth-beats (regular polling of the other process to see whether it's still alive), or using interrupts sent from the other process itself, which will trigger a timeout if it has crashed.
You could use sockets to make the connection between the watchdog and your app work, for example.
It's becoming clear that you need some locking mechanism to prevent swapping to the pagefile / swap-partition. On Posix Systems, this can be done by the m(un)lock* family of functions.
There's a problem with deleting the file. It's not really gone.
When you delete files off your hard drive (not counting the recycle bin) the file isn't really gone. Just the pointer to the file is removed.
Ever see those spy movies where they overwrite the hard drive 6, 8,24 times and that's how they know that it's clean.. Well they do that for a reason.
I'd make every effort to not store the file's decrypted data. Or if you must, make it small amounts of data. Even, disjointed data.
If you must, then they try catch should protect you a bit.. Nothing can protect from the power outage though.
Best of luck.
Check out tmpfile().
It is part of BSD UNIX not sure if it is standard.
But it creates a temporary file and automatically unlinks it so that it will be deleted on close.
Writing to the file system (even temporarily) is insecure.
Do that only if you really have to.
Optionally you could create an in-memory file system.
Never used one myself so no recommendations but a quick google found a few.
In C++ you should use an RAII tactic:
class Clean_Up_File {
std::string filename_;
public Clean_Up_File(std::string filename) { ... } //open/create file
public ~Clean_Up_File() { ... } //delete file
}
int main()
{
Clean_Up_File file_will_be_deleted_on_program_exit("my_file.txt");
}
RAII helps automate a lot of cleanup. You simply create an object on the stack, and have that object do clean up at the end of its lifetime (in the destructor which will be called when the object falls out of scope). ScopeGuard even makes it a little easier.
But, as others have mentioned, this only works in "normal" circumstances. If the user unplugs the computer you can't guarantee that the file will be deleted. And it may be possible to undelete the file (even on UNIX it's possible to "grep the harddrive").
Additionally, as pointed out in the comments, there are some cases where objects don't fall out of scope (for instance, the std::exit(int) function exits the program without leaving the current scope), so RAII doesn't work in those cases. Personally, I never call std::exit(int), and instead I either throw exceptions (which will unwind the stack and call destructors; which I consider an "abnormal exit") or return an error code from main() (which will call destructors and which I also consider an "abnormal exit"). IIRC, sending a SIGKILL also does not call destructors, and SIGKILL can't be caught, so there you're also out of luck.
This is a tricky topic. Generally, you don't want to write decrypted files to disk if you can avoid it. But keeping them in memory doesn't always guarentee that they won't be written to disk as part of a pagefile or otherwise.
I read articles about this a long time ago, and I remember there being some difference between Windows and Linux in that one could guarentee a memory page wouldn't be written to disk and one couldn't; but I don't remember clearly.
If you want to do your due diligence, you can look that topic up and read about it. It all depends on your threat model and what you're willing to protect against. After all, you can use compressed air to chill RAM and pull the encryption key out of that (which was actually on the new Christian Slater spy show, My Own Worst Enemy - which I thought was the best use of cutting edge, accurate, computer security techniques in media yet)
on Linux/Unix, use unlink as soon as you created the file. The file will be removed as soon as you program closes the file descriptor or exits.
Better yet, the file will be removed even if the whole system crashes - because it is basically removed as soon as you unlink it.
The data will not be physically deleted from the disk, of course, so it still may be available for hacking.
Remember that the computer could be powered down at any time. Then, somebody you don't like could boot up with a Linux live CD, and examine your disk in any level of detail desired without changing a thing. No system that writes plaintext to the disk can be secure against such attacks, and they aren't hard to do.
You could set up a function that will overwrite the file with ones and zeros repeatedly, preferably injecting some randomness, and set it up to run at end of program, or at exit. This will work, provided there are no hardware or software glitches, power failures, or other interruptions, and provided the file system writes to only the sectors it claims to be using (journalling file systems, for example, may leave parts of the file elsewhere).
Therefore, if you want security, you need to make sure no plaintext is written out, and that also means it cannot be written to swap space or the equivalent. Find out how to mark memory as unswappable on all platforms you're writing for. Make sure decryption keys and the like are treated the same way as plaintext: never written to the disk under any circumstances, and kept in unswappable memory.
Then, your system should be secure against attacks short of hostiles breaking in, interrupting you, and freezing your RAM chips before powering down, so they don't lose their contents before being transferred for examination. Or authorities demanding your key, legally (check your local laws here) or illegally.
Moral of the story: real security is hard.
The method that I am going to implement will be to stream the decryption- so that the only part that is in memory is the part that is decrypted during the read as the data is being used. Here is a diagram of the pipeline:
This will be a streamed implementation, so the only data that is in memory is the data that I am consuming in the application at any given point. This makes some things tricky- considering a lot of traditional file tricks are no longer available, but since the implementation will be stream based i will still be able to seek to different points of the file which would be translated to the crypt stream to decrypt at different sections.
Basically, it will be encrypting blocks of the file at a time - so then if I try to seek to a certain point it will decrypt that block to read. When I read past a block it decrypts the next block and releases the previous (within the crypt stream).
This implementation does not require me to decrypt to a file or to memory and is compatible with other stream consumers and providers (fstream).
This is my 'plan'. I have not done this type of work with fstream before and I will likely be posting a question as soon as I am ready to work on this.
Thanks for all the other answers- it was very informative.